This application claims the priority of German Patent Application, Serial No. 100 64 833.9, filed Dec. 22, 2000, and German Patent Application, Serial No. 101 25 691.4, filed May 25, 2001, the subject matter of which is incorporated herein by reference.
The present invention relates to a release bearing which, when installed in a motor vehicle, is intended for actuation of a separating clutch disposed between the internal combustion engine and a gearbox.
Typically, a release bearing includes a first, non-rotatable bearing ring, a second revolving bearing ring, and rolling elements, received in a cage and guided between the first and second bearing rings. The bearing rings are normally made without material removal through a deep-drawing process and formed with an annular bottom, which is oriented, when installed, at a right angle to a symmetry axis of the release bearing. The annular bottom of the non-rotatable bearing ring is supported, when installed, by an axially shiftable casing which is operatively connected with an actuation device. The annular bottom of the revolving bearing ring is operatively connected with a spring member, in particular arms of a disk spring of the separating clutch.
Operation of the separating clutch is typically implemented by depressing a clutch pedal inside the motor vehicle. Upon depression of the pedal, transmission elements or fluids shift the release bearing against the spring member of the separating clutch, in particular the arms of the disk spring. During operation, a relative movement in radial direction is encountered between the disk spring and the annular bottom of the revolving bearing ring in the contact area of the disk spring and the annular bottom of the bearing ring. Superimposed movements as a result of the axial impact of the disk spring arms and axial oscillations triggered by the internal combustion engine via the separating clutch lead to severe wear in the presence of a purely metallic contact of the disk spring upon the annular flange of the bearing ring. This support further results in an annoying noise and increases the required pedal forces for depression of the clutch pedal, thereby adversely affecting the overall driving comfort.
Various attempts have been undertaken to reduce wear in the contact area. German Pat. No. DE 22 40 383 A1 discloses a release bearing with bearing rings that have been made through a material removal process. The revolving inner bearing ring forms a contact surface which points toward the separating clutch. An attachment ring is supported by this contact surface and is placed between the disk spring of the separating clutch and the release bearing for compensating an axial offset between the rotating components of the separating clutch and the rotating components of the actuating device. When installed, the attachment ring allows a relative axial movement in relation to the radial contact surface of the revolving bearing ring. The contact surface between the frictional components, i.e. the attachment ring and the support surface, is lined with a friction-reducing coating. In order to restrict the radial mobility of the attachment ring, when the separating clutch is engaged, the attachment ring has an outer axial collar on the outside spaced in radial direction to the outer contour of the contact surface of the bearing ring. As a consequence, the collar encircles the contact surface. A sheet metal disk is attached on the free collar end, which points toward the release bearing and radially overlaps the outer contour of the contact surface of the revolving bearing ring to thereby captivate the attachment ring.
This conventional release bearing suffers shortcomings because the floating disposition on the rotating bearing ring does not allow a defined position of the installed attachment ring. A defined state is also not possible because a relative movement between the attachment ring and the disk spring as well as between the attachment ring and the rotating bearing ring cannot be excluded. Moreover, the release bearing requires a relatively large space for installation in axial as well as radial directions as a consequence of the fabrication of the components through a material removing process and the floating disposition of the attachment ring upon the rotating bearing ring.
It would therefore be desirable and advantageous to provide an improved release bearing which obviates prior art shortcomings.
According to one aspect of the present invention, a release bearing for a separating clutch installed in a motor vehicle, includes a casing movable in an axial direction and operatively connected to an actuating mechanism; a non-rotatable bearing ring having a ring bottom supported by the casing; a revolving bearing ring having an annular flange; a plurality of rolling elements guided between the bearing rings; an engagement disk made of a low-friction and wear-resistant material and securely fixed to the annular flange; and a spring member operatively connected to the separating clutch and cooperating with the annular flange of the revolving bearing ring, wherein the spring member bears against the engagement disk.
The present invention resolves prior art problems by providing a wear-reducing, frictionally optimized and compact engagement disk which is yet cost-efficient to manufacture and ensures a long service life demanded by the automobile industry.
Through the provision of the engagement disk which is secured in place and yet centered upon the annular flange of the revolving bearing ring, the relative movement between the spring member, the disk spring of the separating clutch and the revolving bearing ring is established solely between the frictional components, engagement disk and disk spring or its spring arms. At the same time, the rigid disposition of the engagement disk on the rotating bearing ring results in a clear definition of the required installation space. The engagement disk can be made compact in axial as well as radial directions. As the engagement disk can be made of wear-reducing material, required actuation forces or pedal forces are decreased, thereby enhancing the operating and driving comfort and produces a damping effect of the engagement disk. This is especially desired in vehicles of high engine power.
Suitably, the engagement disk is secured to the annular flange of the revolving bearing ring through a form-fitting (positive) connection. According to one embodiment of the present invention, the engagement disk may be formed about its circumference with collar segments which engage behind or embrace the annular flange for attachment of the engagement disk to the bearing ring. The collar segments may be formed with holding elements configured as retaining lugs for engagement in radial cutouts or recesses of the annular flange to thereby realize a snap-connection of the engagement disk upon the annular flange.
According to another embodiment of the present invention, the engagement disk may have axially projecting cams or protrusions for engagement in complementary recesses of the annular flange, wherein the cams allow a limited rotation of the engagement disk with respect to the annular flange and thereby secure the engagement disk against rotation in end position with respect to the revolving bearing ring. This type of safeguard against rotation can also be implemented by a bayonet connection and can be so configured that the cams of the engagement disk as well as the recesses in the annular flange of the revolving bearing ring can be integrated in the fabrication process of the component, without increasing the manufacturing costs. The recesses, which permit a limited rotation of the engagement disk in relation to the annular flange, are so arranged that the cams seek the holding position during operation of the internal combustion engine so as to prevent a spontaneous detachment.
According to another feature of the present invention, the engagement disk may be secured to the revolving bearing ring by a force-fitting (non-positive) connection. One option includes the provision of a high-temperature glue which can be applied partially or completely onto the contact surface between the engagement disk and the annular flange. Of course a combination of form-fitting connection and force-fitting connection between the engagement disk and the annular flange of the revolving bearing ring may also be conceivable.
According to another feature of the present invention, the engagement disk and the revolving bearing ring are made of different materials. Material choice may depend on optimum strength, wear behavior or friction conditions. For example, the revolving bearing ring may be made of steel, e.g., in a cost-efficient manner through a shaping process without material removal, such as through a deep drawing process.
When the engagement disk is made of a material which differs from the material of the bearing ring so that varying coefficients of expansion are involved in such a combination, particular consideration is required in conjunction with the attachment of the engagement disk upon the annular flange. According to another feature of the present invention, the annular flange may hereby be provided about its circumference with radial cutouts for form-fitting engagement of axial cams, protrusions and/or collar segments of the engagement disk. During installation, a plastic deformation occurs, whereby reliefs positioned adjacent both sides of the cutouts are at least partially filled by the material of the cams. In this way, this type of securement allows a radial movement of the axial cams or protrusions of the engagement disk in relation to the annular flange in the area of the radial cutouts. This is true even when the engagement disk is fixed in place to the bearing ring, i.e., a relative displacement between the annular flange and the engagement disk is possible as a consequence of the particular coefficients of expansion in view of the different materials. In this way, tension, which would adversely affect the fatigue strength of the engagement disk, is avoided.
It is also conceivable to fabricate the engagement disk of different materials. Examples of suitable materials include steel, thermoplastic and duroplastic as well as ceramic. In the event the engagement disk is made of steel, at least the area of the contact surface for support of the spring member, connected to the separating clutch and comprised of the disk spring with the elastic spring arms, is lined with a sliding-enhancing coating or a friction-reducing coating. Examples includes a PTFE coating or also molybdenum sulfide (MOS2) or a PVD coating. The use of an engagement disk of steel has the advantage that the bearing ring has a same coefficient of expansion as the engagement surface so that tension between the bearing ring and the engagement disk is avoided. The modulus of elasticity of steel further allows an elastic snap-connection of the engagement disk with the annular flange of the revolving bearing ring through provision of retention elements, thereby simplifying the assembly.
According to another feature of the present invention, the engagement disk may be made of plastic, e.g. a duroplastic compound containing suitable additives. Friction and wear as well as strength can be positively influenced, when using as additive a phenol-formaldehyde (PF) containing glass fibers and/or carbon fibers, as well as graphite and/or molybdenum sulfide and/or polyphenylene sulfone (PPSO2).
The engagement disk may also be made of thermoplastic material, e.g. based on a highly heat-resistant polyamide (PA46) or polyaryletherketone (PAEK). These type of plastics may contain suitable additives such as carbon fibers and/or aramide fibers for enhancing the strength and improving wear resistance. Additives such as molybdenum sulfide or graphite and/or polytetrafluoroethylene as well as polyphenylene sulfone are suitable to reduce friction. An injection molding process is suitably employed, when making the engagement disk of plastic material.
According to another feature of the present invention, the engagement disk may be spray-coated as a coating directly onto the annular flange of the revolving bearing ring. A centered disposition of the engagement disk can be realized by extending the coating to the inner circumference of the annular bottom of the revolving bearing ring. Another option to implement an immobile disposition of the engagement disk upon the annular bottom includes the application of the coating such that the coating at least partially encircles the outer contour of the annular flange.
According to another feature of the present invention, the engagement disk may be made of ceramic. Ceramic has a high strength and thus is effectively wear-resistant. Like duroplastic, ceramic has a lower modulus of elasticity than steel so that the provision of an elastic snap-on connection of the engagement disk via retention elements onto the annular flange of the revolving bearing ring is precluded. This type of an engagement disk can then be secured to the annular flange by a form-fitting connection in the form of a bayonet type coupling. A possibility of such a form-fitting connection involves insertion of cams, formed in one piece with the engagement disk and having retaining lugs, in axial recesses or reliefs of the annular flange. Through subsequent turning of the engagement disk, the retaining lugs are seated firmly in the recesses so as to secure the engagement disk in place with respect to the annular flange and to safeguard the engagement disk against a rotation. As a consequence, the engagement disk is securely mounted to the annular flange and prevented from detachment during running internal combustion engine.
In order to use release bearings for high rpm in conjunction with elevated operating and ambient temperatures, as encountered, for example, in double-disk clutches, some components of the release bearing may be made of ceramic. Suitable components include the annular flange, the rolling elements as well as the revolving bearing ring. Lubrication of a such a release bearing can be realized by a dry lubricant.